In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, the demand for secondary batteries as built-in batteries of these electronic devices has been increasing. In particular, the development of lithium ion secondary batteries which have a high energy density and are capable of being charged/discharged has been extensively conducted.
Power consumption of portable electronic devices has been remarkably increasing as the number of their functions has been increased. For coping with the increase in power consumption, lithium ion secondary batteries having a large capacity have been required.
In the lithium ion secondary battery, in general, a metal oxide such as lithium cobaltate is used as a positive electrode active material, a carbon material such as graphite is used as a negative electrode active material, and a solution obtained by dissolving lithium hexafluorophosphate in an organic solvent, i.e. an organic solvent-based electrolytic solution is used as an electrolyte. In a battery having the above-mentioned configuration, an attempt has been made to increase internal energy by increasing the amount of an active material, and further, enhance the energy density, so that the output current is improved. It is also expected that the battery is increased in size and that the battery is mounted on a vehicle.
However, in a lithium ion secondary battery having the above-described configuration, an organic solvent used for the electrolyte is a combustible material, and therefore there is the risk that the battery may catch fire. For this reason, it is required that safety of the battery be further enhanced.
One of measures for enhancing safety of the lithium ion secondary battery is to use a solid electrolyte in place of an organic solvent-based electrolytic solution. As the solid electrolyte, use of organic materials such as polymers and gels and inorganic materials such as glass and ceramic is being studied. In particular, an all-solid secondary battery including as a solid electrolyte an inorganic material having incombustible glass or ceramic as a main component has been proposed, and is attracting attention.
For example, Japanese Patent Laid-open Publication No. 2003-68361 (hereinafter, referred to as Patent Document 1) describes a configuration of an all-solid lithium secondary battery including an incombustible solid electrolyte. In the all-solid lithium secondary battery, the solid electrolyte is a material which has a sulfide as a basic composition, and is composed of lithium sulfide and phosphorus sulfide, or a material which is composed principally of lithium sulfide and phosphorus sulfide, does not contain a transition metal element and does not contain silicon and germanium, a negative electrode active material is a carbon material, or a material with lithium ions inserted between carbon material layers, and a positive electrode active material is lithium cobaltate, lithium nickelate, lithium manganate or the like. Patent Document 1 describes that when graphite is used as a negative electrode active material, battery characteristics significantly vary depending on the type of the solid electrolyte, and selection of a lithium ion-conductive solid electrolyte is important for preparing an all-solid lithium secondary battery having excellent performance. It is described that the energy density of the all-solid lithium secondary battery can be enhanced when a sulfide which does not contain silicon and germanium is used as a solid electrolyte based on this consideration.
For example, Japanese Patent Laid-open Publication No. 2008-288098 (hereinafter, referred to as Patent Document 2) describes that a powder having an average particle size of 0.01 to 10 μm is used as a sulfide-based solid electrolyte having a high ion conductivity.
Patent Document 1: Japanese Patent Laid-open Publication No. 2003-68361
Patent Document 2: Japanese Patent Laid-open Publication No. 2008-288098